To study the dynamic process excited by femtsecond laser,a carrier model with three energy level and trap is constructed,with which the electron rate equation is obtained.The numerical simulation is used to calculate the electron number change of each energy level and the differentiated absorbtion rate as the time goes by.With the change of the electron’s absorption cross-section,differentiated absorbtion rate will have a ultrafast change process.Compared with the experimental result of Fanxin Wu,the differentiated absorbtion rate curves are basically the same,which shows the model have some rationality.